1. Field
The present invention relates to a process and an apparatus for producing pneumatic tyres.
2. Description of the Related Art
A tyre for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures integrated into the regions usually identified as “beads”.
Associated with the carcass structure is a belt structure comprising one or more belt layers located in radially superposed relationship with respect to each other and to the carcass ply and having textile or metallic reinforcing cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. A tread band is applied to the belt structure at a radially external position, which tread band too is made of elastomeric material, like the other semifinished products constituting the tyre.
It is to be pointed out herein that, to the aims of the present description, by the term “elastomeric material” it is intended a compound comprising at least one elastomeric polymer and at least one reinforcing filler. Preferably, this compound further comprises additives such as cross-linking agents and/or plasticizers, for example. Due to the presence of the cross-linking agents, this material can be cross-linked by heating, so as to form the final manufactured product.
Respective sidewalls of elastomeric material are also applied to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads. In tyres of the tubeless type, an air-tight coating layer usually referred to as “liner”, covers the inner tyre surfaces.
Generally, manufacture of tyres for vehicle wheels contemplates, after building of the green tyre through assembling of the respective components, a moulding and vulcanising treatment to be carried out for the purpose of determining the structural stabilisation of the tyre through cross-linking of the elastomeric compounds and forming a desired tread pattern thereon as well as printing possible distinctive graphic signs at the tyre sidewalls.
Producing processes are known in which building of the green tyre is carried out through manufacture of the different components thereof by laying series of basic components on a sufficiently rigid toroidal support the conformation of which matches the inner conformation of the finished tyre, which basic components consist of rubber-coated cords, strip-like elements of rubber-coated cords, and/or elongated elements of elastomeric material wound up around the toroidal support through a so-called operation to form circumferential coils disposed in mutual side by side relationship. To the aims of the moulding and vulcanising treatment, the green tyre together with the toroidal support on which said tyre has been built, is enclosed into the moulding cavity of a vulcanisation mould the shape of which matches the outer configuration to be given to the finished tyre.
Generally, in vulcanisation processes the steam under pressure admitted into the toroidal support determines supply of part of the necessary heat for carrying out vulcanisation. Another part of the necessary heat is usually supplied through the mould from the outside of the tyre, suitably heated by means of channels provided in the vulcanisation apparatus, through which steam or other heating fluid runs.
WO-01/00395 in the name of the same Applicant discloses a process of the above mentioned type involving use of a toroidal support of an outer diameter slightly smaller than the inner diameter of the finished tyre. After the tyre built on the toroidal support has been enclosed into the vulcanisation mould, it is pressed against the holding walls of the moulding cavity while steam under pressure is admitted into the toroidal support. The crown region of the tyre is moulded against the inner surface, of the moulding cavity following a radial expansion induced by steam under pressure that will fill a diffusion interspace defined between the toroidal support and the inner surface of the tyre itself.
U.S. Pat. No. 5,853,526 discloses a building process in which the tyre components are formed on an expandable toroidal support including a reinforced bladder the inner end flaps of which are sealingly fastened to mutually coaxial anchoring flanges integral with two half-shafts telescopically in engagement with each other. The reinforced bladder inflated to a predetermined pressure, keeps a predefined geometric structure corresponding to the inner conformation of the tyre to be built, so that it lends itself to support the components of the tyre being processed. When building has been completed, the tyre is enclosed into the vulcanisation mould together with the expandable toroidal support. The bladder will receive steam to determine pressing of the tyre against the inner surfaces of the moulding cavity and simultaneous heat supply to the tyre itself, for vulcanisation.
WO-2004/045837 in the name of the same Applicant as well, proposes carrying out of a preliminary heat supply to the tyre through feeding of a counterpressure fluid into the mould, externally of the tyre itself, to counteract pressure of the steam or other fluid fed to the inside of the toroidal support. Thus heat can be supplied to a sufficiently high temperature and for a period of time long enough to ensure correct consolidation or strengthening of the beads and sufficient cross-linking of the liner, before moulding of the tyre crown portion is carried out through admission of high-pressure steam into the diffusion interspace.
On practically carrying out the above processes, the Applicant has encountered different difficulties correlated with the tyre moulding and vulcanising treatment.
In particular, the Applicant could observe some difficulties for obtaining a constant quality of the product at the beads. In fact, even when particular attention is paid during the tyre building steps so as to manufacture the tyre components with precise shape and size tolerances, often structural faults are encountered at the beads at the end Of the moulding and vulcanisation steps.
The Applicant has noticed that one of the possible causes for these faults is to be sought in the sudden unavoidable deformations imposed to the elastomeric tyre components during the step of closing the mould. In fact, it has been found that the individual components of the green tyre, above all those made through a spiralling operation, do not have a conformation exactly identical with that imposed by the mould to the finished product. On closure of the mould, the elastomeric material composing the tyre is forced to adapt itself in an almost instantaneous manner to the inner conformation of the mould, above all at the beads that are usually the first tyre regions coming into contact with the inner walls of the mould on closure. In addition, since the beads must have a high geometric and size precision, they are usually held between the inner mould walls and the toroidal support to be submitted to a moulding operation of the so-called “imposed-volume” type, in which migration of possible elastomeric excess material to the sides or other parts of the mould cavity is imposed.
The tyre is therefore submitted to sudden deformations transmitting anomalous and hardly controllable stresses to the different structural components of the beads, so that undesirable deformations can be caused.
The Applicant has therefore perceived that by blocking the geometry of the green tyre beads at the end of the building process and keeping this blocking during the moulding and vulcanisation steps, the above mentioned drawbacks can be overcome and a finished product can be obtained that is more in compliance with the design parameters.